By Olivia Walch

Today’s guest writer is Olivia Walch. She’s the founder of Arcascope and the author of the forthcoming book The Sleep Groove, which is available for preorder.

Long, Long Sleeps

What happens if you put people who’ve been sleeping 7 to 8 hours a night in total darkness for 14 hours a day for a month? Answer: The first night, they sleep a long time — close to 12 hours. The next night, they sleep about 10-ish hours, then 9.5-ish, then after a week it’s down to 9 hours, before eventually leveling out close to 8 hours near the end of the month of long nights. You can think of this gradual drop-off in sleep duration as capturing a washing-out of sleep debt. And if people sleeping 7 to 8 hours a night had sleep to catch up on, surely the rest of us are doomed. Ah, if only modern life weren’t ruining things for us, we’d all be sleeping the long, long sleeps we’re designed for. 

Wehr et al., 1993

Not So Long Sleeps

Except if we look at people living in pre-industrial societies, they aren’t usually sleeping 8 hours a night. In one study of three societies without artificial light at night, sleep durations ranged from 5.7 to 7.1 hours a night — pretty typical of what we see in modern life. This sparked a spirited back-and-forth in the literature, which I’ll summarize as “other similar societies do sleep closer to 8 hours” and “just because people in a pre-industrial society do something doesn’t mean that thing is healthy.” Still, only 1.5 percent and 2.5 percent of the people surveyed in the original study said they had regular problems with sleep onset and sleep maintenance, which is a lot lower than what you’d get if you asked the same question to people living in the U.S. today. 

Yetish et al., 2015

So What’s Different?

One big difference between societies without artificial light and how we live now is (surprise, surprise) they get way less light at night than we do. This has ramifications for our body’s natural production of melatonin, which gets squashed in the presence of light. In 2020, researchers reported that nearly half of typical homes had evening light levels that would be bright enough to suppress natural melatonin production by 50 percent. 

Cain et al., 2020

More About Melatonin

In a lot of ways, melatonin is “anti-light.” It’s the hormonal cue for nighttime. Your body tries to make melatonin on its own, once a day, but it’ll suppress production if the lights are too bright. The market for melatonin is huge — essentially $1 billion — and 46 percent of parents report giving melatonin supplements to their kids under 13. All of this (pre-industrial societies reporting lower rates of insomnia despite not sleeping longer, homes being bright enough to suppress melatonin, parents giving kids melatonin) suggests that we’re prooobably getting too much light at night.  

American Academy of Sleep Medicine

Why That’s Bad

It’s not just that we’re making less melatonin on our own. Light at night confuses the clocks in our body — our circadian rhythms. And circadian rhythms impact a lot more than just our sleep. Just one night of keeping the light on was found to increase nighttime heart rate, decrease heart rate variability, and increase insulin resistance the next morning in otherwise healthy adults. Unrelatedly, I’ve now set up the guest room in my house so that I can turn the lights off remotely when Walter visits.

Mason et al., 2022

Why That’s Bad II

Scale up a single night with too much light to a lifetime’s worth of light at night and you start to understand why people with higher sleep regularity (and more consistent light exposure) have a “20 percent to 48 percent lower risk of all-cause mortality, a 16 percent to 39 percent lower risk of cancer mortality, and a 22 percent to 57 percent lower risk of cardiometabolic mortality” than people in the most irregular sleep cohort. Regular sleep means your brain’s more confident about what time it is. Irregular sleep makes it lose confidence, which throws off rhythms all throughout your body. 

Windred et al., 2024

Beyond Sleep Duration

We normally think about sleep health as “getting 7 to 9 hours of sleep a night,” but more and more we’re realizing that we need to care about when those hours of sleep happen. In that last study, sleep regularity was found to be a better predictor of mortality than sleep duration, while another study looking at factors that predict mortality in older men found sleep rhythmicity to be the highest ranking sleep-related factor, ranking fifth. Sleep duration, on the other hand, only showed up about 18 spots further down the list. Does this mean sleep duration doesn’t matter? Of course not. Does it mean we need a definition of sleep health that includes regularity and rhythmicity? Sure seems that way. 

Wallace et al., 2018

Preorder The Sleep Groove today!

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